1. Field of the Invention
The present invention relates generally to image projection apparatuses implemented with mirror devices functioning as a spatial light modulator (SLM). The present invention relate more particularly to an image projection apparatus implemented with mirror devices functioning as SLM to receive illumination light from a light source that includes a light control unit to modulate the illumination light to control the gray scale of an image projection with non-linear control processes.
2. Description of the Related Art
Even though there have been significant advances in recent years in the technology of implementing electromechanical micro-mirror devices as spatial light modulators, there are still limitations and difficulties when these are employed to display high quality images. Specifically, when an image is digitally controlled, the image quality is adversely affected because the image is not displayed with a sufficient number of gray scales.
Electromechanical micro-mirror devices have drawn considerable interest because of their application as spatial light modulators (SLMs). A spatial light modulator requires a relatively large number of micro-mirror devices. In general, the number of devices required ranges from 60,000 to several million for each SLM. Refer to FIG. 1A for a digital video system 1 as disclosed in a relevant U.S. Pat. No. 5,214,420, which includes a display screen 2. A light source 10 is used to generate light energy to illuminate display screen 2. Light 9 is further concentrated and directed toward lens 12 by mirror 11. Lens 12, 13, and 14 serve a combined function as a beam columnator to direct light 9 into a column of light 8. A spatial light modulator 15 is controlled by a computer through data transmitted over data cable 18 to selectively redirect a portion of the light from path 7 toward lens 5 to display on screen 2. The SLM 15 has a surface 16 that includes switchable reflective elements, e.g., micro-mirror devices 32 with elements 17, 27, 37, and 47 as reflective elements attached to a hinge 30, as shown in FIG. 1B. When element 17 is in one position, a portion of the light from path 7 is redirected along path 6 to lens 5 where it is enlarged or spread along path 4 to impinge the display screen 2 so as to form an illuminated pixel 3. When element 17 is in another position, light is not redirected toward display screen 2 and hence pixel 3 would be dark.
The on-and-off states of a micro-mirror control scheme, such as that implemented in the U.S. Pat. No. 5,214,420 and by most conventional display systems, limits image display quality. This is because the application of a conventional control circuit limits the gray scale (PWM between ON and OFF states) by the LSB (least significant bit, or the least pulse width). Due to the ON-OFF states implemented in conventional systems, there is no way to provide a pulse width shorter than the LSB. The least brightness, which determines the gray scale, is the light reflected during the least pulse width. A limited gray scale leads to lower image quality.
In FIG. 1C, a circuit diagram of a control circuit for a micro-mirror according to U.S. Pat. No. 5,285,407 is presented. The control circuit includes memory cell 32. Various transistors are referred to as “M*” where * designates a transistor number and each transistor is an insulated gate field effect transistor. Transistors M5, and M7 are p-channel transistors; transistors, M6, M8, and M9 are n-channel transistors. The capacitances, C1 and C2, represent the capacitive loads presented to memory cell 32. Memory cell 32 includes an access switch transistor M9 and a latch 32a, which is the basis of the static random access switch memory (SRAM) design. All access transistors M9 in a row receive a DATA signal from a different bit-line 31a. The particular memory cell 32 to be written is accessed by turning on the appropriate row select transistor M9, using the ROW signal functioning as a word-line. Latch 32a is formed from two cross-coupled inverters, M5/M6 and M7/M8, which permit two stable states. State 1 is Node A high and Node B low and state 2 is Node A low and Node B high.
The switching of the dual states, as illustrated by the control circuit, controls the micro-mirrors to position either at an ON or an OFF angular orientation, as shown in FIG. 1A. The brightness, i.e., the gray scales of display for a digitally control image system, is determined by the length of time the micro-mirror stays at an ON position. The length of time a micro-mirror is at an ON position is, in turn, controlled by a multiple bit word. FIG. 1D shows the resultant binary time intervals when the device is controlled by a four-bit word. As shown in FIG. 1D, the time durations have relative values of 1, 2, 4, and 8 that in turn define the relative brightness for each of the four bits where 1 is for the least significant bit and 8 is for the most significant bit. According to the control mechanism as shown, the minimum controllable differences between gray scales for showing different brightness is a brightness represented by the “least significant bit” that keeps the micro-mirror at an ON position.
For example, assuming n bits of gray scales, the frame time is divided into 2n−1 equal time periods. For a 16.7 milliseconds frame period and n-bit intensity values, the time period is 16.7/(2n−1) milliseconds
Having established these time slices for controlling the length of time for displaying each pixel in each frame, the pixel intensities are determined by the number of time slices represented by each bit. Specifically, a display of a black pixel is represented by 0 time slices. The intensity level represented by the LSB is 1 time slice, and maximum brightness is 2n−1 time slices. The number time slices that a micro mirror is controlled to operate at an On-state in a frame period determines a specifically quantified light intensity of each pixel corresponding to the micromirror reflecting a modulated light to that pixel. Thus, during a frame period, each pixel corresponding to a modulated micromirror controlled by a control word with a quantified value of more than 0 is operated at an on state for the number of time slices that correspond to the quantified value represented by the control word. The viewer's eye integrates the pixel brightness so that the image appears the same as if it were generated with analog levels of light.
For addressing deformable mirror devices, a pulse width modulator (PWM) receives the data formatted into “bit-planes”. Each bit-plane corresponds to a bit weight of the intensity value. Thus, if each pixel's intensity is represented by an n-bit value, each frame of data has n bit-planes. Each bit-plane has a 0 or 1 value for each display element. In the example described in the preceding paragraphs, each bit-plane is separately loaded during a frame. The display elements are addressed according to their associated bit-plane values. For example, the bit-plane representing the LSBs of each pixel is displayed for 1 time period.
Projection apparatuses such as described above generally use a light source such as a high-pressure mercury lamp, a xenon lamp, or similar kinds of light sources. However, these types of light sources perform poorly in high-speed switching that alternates between the ON and OFF states. Accordingly, a light source is usually controlled to continuously operate in an ON state during the entire length of time when the projection apparatus is in operation. The light source that is continuously turned on generates a large amount of heat and wastes light and electricity.
There is also an increasing demand that projection apparatuses project images at a higher level of gray scale (gradation). Accordingly, a spatial light modulator has to be controlled to enable a projection apparatus to project images at a higher level of gray scale. However, the achievable improvement of the gray scale performance would be very limited if the improvements are to be achieved only through the control of the spatial light modulator limited. Some of the attempts to further improve the quality of image display have been disclosed in many Patents, such as U.S. Pat. Nos. 5,214,420, 5,285,407, and published Patent Applications. However, the disclosures including those included in the Information Disclosure Statement (IDS) have not provided effective solutions to resolve the above discussed difficulties and limitations.
Recent developments in image display technology; a so-called γ correction is performed when the cameras capture the images. The γ correction is carried out in order to take into account the projection characteristics of the CRT devices for displaying the image as the TV images projected from the CRT devices.
Therefore, the signal voltages E applied in the CRT devices for projecting the TV images have a functional relationship with the image projection output L represented by L=Eγ. In other words, the relationship when multiplied by a non-linear γ correction is non-linear. Also, in order to reduce the cost to the TV receivers as consumers, this γ correction is performed on the transmission side when image data is generated.
In contrast, unlike the CRT devices, the display characteristic of the projection apparatuses using micro-mirror devices as described above is linear. Accordingly, it is necessary to perform a reverse correction on broadcasted image signals in order to cancel the γ correction performed on the transmission side.
An example of the γ correction performed on the reception side is one in which a prescribed mathematical operation is performed on the input data itself. However, this mathematical operation for the γ correction is complicated because its use of a logarithm function. Also, a larger scale operation circuit is required, which to increases the production costs of projection apparatuses.
It is also possible to adapt a conversion technology using a lookup table or the like, thereby avoiding such mathematical operations. However, in order to attain an acceptable accuracy of operation (i.e., conversion accuracy), the gray scale accuracy of the input data has to be increased (in other words, the number of bits has to be increased) before conversion, which forces the lookup table or the like to occupy a greater volume of memory, which increases the production costs of projection apparatuses.